A torando travels across the horizon on May 23, 2008, near Quinter, Kan. / AP/Steven Hausler, The Hays Daily News

by Doyle Rice, USA TODAY

by Doyle Rice, USA TODAY

The USA in the past 12 months has seen the fewest number of tornadoes since at least 1954, and the death tolls from the dangerous storms have dropped dramatically since 2011.

Just two years after a ferocious series of tornado outbreaks killed hundreds of Americans, the USA so far this year is enjoying one of the calmest years on record for twisters. Through Thursday, tornadoes have killed only three Americans in 2013; by the end of May 2011, 543 Americans had died.

The seven people killed from May 2012 to April 2013 is the fewest in a 12-month period since five people died in September 1899-August 1900, according to Harold Brooks, research meteorologist with the National Severe Storms Laboratory in Norman, Okla.

The year-to-date count of tornadoes is probably approaching the lower 10% of all years on record, said Greg Carbin, warning coordination meteorologist with the Storm Prediction Center in Norman.

The reason: An unusually cool weather pattern from the Rockies to the East Coast. "Generally, the lower the temperature and/or the drier the air, the lower the number of thunderstorms," said AccuWeather meteorologist Alex Sosnowski.

Severe thunderstorms produce tornadoes, along with large hail and high winds.

So far in May - usually the USA's most active month - only three tornadoes have formed. All have been rated EF-0 on the Fujita scale of tornado intensity. EF-0 is the weakest rating for tornadoes, with wind speeds of about 65-85 mph.

The EF-5 tornado that ravaged Joplin, Mo., two years ago, had estimated wind speeds as high as 250 mph and killed 158 people.

The record low in tornadoes comes less than two years after a record high from 2010 to 2011, Weather Underground meteorologist Jeff Masters said. "The extraordinary contrast underscores the crazy fluctuations we've seen in Northern Hemisphere jet stream patterns during the past three years," he said. "Call it 'weather whiplash' of the tornado variety."

Current weather patterns are expected to continue into the first part of summer, likely keeping 2013 well behind the curve for violent thunderstorms and tornadoes, AccuWeather reports.